Production of colloidal silver



Patented Oct. 16, 1951 PRODUCTION COLLOlbAL SILVER Harold OwenDickinson, Ilford, England, assignor to Ilford Limited, Ilford, England,a British company NoDrawing. Application November 19, 1947, Serial No.787,025. In Great Britain November iifCThis invention relates to theproduction of colloidal silver and to the use of layers of suchcolloidal silver in colloid media in photographic materials. It has longbeen knownthat by the addition of a reducing agent to a dispersion of asoluble silver salt in a colloid medium such as gelatin, there is slowlyformed a colloidal dispersion of silver which is, in most cases, yellowin colour. Thus, one method which has been proposed is to add to adispersion of silver nitrate in gelatin a quantity of hydrazinesulphate. The hydrazine sulphate is slowly added over a period of onehour and the resulting mixture allowed to stand at a slightly elevatedtemperature for a further three-quarters of an hour. In this way ayellow dispersion of colloidal silver is obtained.

Yellow colloidal silver thus formed is of value in photography as amedium for filter and anti halation layers and various processesemploying yellow colloidal silver for these purposes have been describedin the literature.

The present invention is concerned with the production of colloidalsilver which may be employed in the production of filter andanti-halation layers in photographic materials, and is based on thediscovery that if there is present during the reaction a smallproportion of silver sulphide or silver selenide nuclei, i. e. colloidalsilver sulphide or silver selenide, the reaction proceeds at a very muchaccelerated rate, and yellow colloidal silver of excellent quality isobtained.

According to the present invention, therefore, colloidal silver isobtained by reacting in an aqueous colloidal medium a soluble salt ofsilver and a reducing agent therefor, in the presence of dispersedcolloidal silver sulphide or silver selenide.

Whilst the process can be carried out in any water-permeable colloidalmedium, e. g. an aqueous dispersion of polyvinyl alcohol or similarsubstance, it is preferred to efiect the reaction in a gelatin medium.The silver salt employed may be any silver salt which is soluble inwater and relatively easily reduced; silver nitrate appears to beparticularly suitable. Whilst any reducing agent may be employed,. it ispreferred to use a reducing agent whichis a good photographic developer,e. g. hydroquinone, p-hydroxyphenylamino-acetic acid,p-methyl-amino-phenol-su1- phate, l-phenyl-pyrazolidone-3,p-phenylenediamine and substitution derivatives. Relatively inefficientphotographic developers, such as hydrazine, are less satisfactory.Particularly good results have been obtained with p-hydroxyphenylaminoacetic acid.

2 Claims. (Cl. 252-313) The colloidal silver sulphide may be obtainedquite readily by reaction between thiourea and silver nitrate. Thepreferred technique is to form the colloidal silver sulphide in thecolloidal medium as a first step and then to add the soluble silversalt, and finally the reducing agent. The resulting dispersioncontaining colloidal silver may then be set, shredded, washed andredispersed in water, forming a coating solution from which filter andanti-halation layers of intense yellow colour and excellent quality maybe obtained.

Colloidal silver selenide nuclei may be prepared by a precisely similartechnique using selenourea instead of thiourea.

The quantities of the reagents and of the silver sulphide or selenide(which appears to act as a catalyst) may be varied widely, but theoptimum quantity of silver nitrate appears to be of the order of 2 cc.of normal silver nitrate solution per cc. of 5% gelatin solution.Smaller quantities than this give a lesser intensity of colour, andquantities below half the above concentration give only faintcolouration. Larger quantities tend to give a rather turbid product, andfor this reason it is preferred not to use more than twice the aboveconcentration. It may be said in general therefore that for mostpurposes the quantity of silver nitrate should be of the order of 1 to 4cc. of normal silver nitrate solution per 100 cc. of 5% gelatinsolution. The quantity of reducing agent employed should besubstantially chemically equivalent to the silver salt in order toobtain the best results. The optimum quantity of silver sulphide orselenide varies with the other factors but, for example, using a 3.5%solution of gelatin the best quantity appears to be that formed by theaddition of 1 cc. of M/20 thiourea or selenourea with 1 cc. of N/ 10silver nitrate per 200 cc. of the gelatin solution. Smaller quantitiesthan this tend to give a somewhat greyish product, while higherquantities, e. g. up to 5 cc. of each component, appear to make nofurther difference to the intensity of the yellow colouration obtained.

The following examples illustrate the invention:

Example 1 2 litres of a 3.5% aqueous solution of gelatin was preparedand to this was added 10 cc. of M/ 20 thiourea and 10 cc. of N/lO silvernitrate. The solution was then digested for 20 minutes at F. and thencooled to 90 F. There was then added 40 cc. of normal silver nitratesolution followed by 400 cc. of N/ 10 p-hydroxy phenyl amino acetic aciddissolved in N/ caustic soda. A strong yellow colouration wasimmediately formed. The gelatin solution was then precipitated withethyl alcohol. It was then soaked for three 10-minute eriods in changesof water, and finally re -disp.ersed in a litreof-v water. This product;coated as a, filter layer'in a multilayer photographic material intendedfor colour photography, was found to have very good filtering;

characteristics and to have very little effect on the speed of thephotographic emulsions with which it was in contact. 7

Example-2* The procedure of Example 1 was followed using an M/ solutionof selenourea instead of thi'o' urea. Precisely similar results wereobtained.

What I claim is: 1. Process for the production of colloidal, sil

ver which comprises reacting in an aqueous gelatin: solution silver;nitrate in the, proportion of volume; of a. normal solution. with /21vol.- umezof al -molar solution of a'compound taken from-the groupconsistingofi thiourealandvselenolurea and adding the colloidal silversalt, formed which is taken: from thewgroup, consisting, of col.-

loidal silver sulfide and colloidal silver selenide to an aqueousgelatin solution containing a quantity of silver nitrate equivalent to 1to 4 volumes of normal silver nitrate solution per volumes of gelatinsolution and admixing a chemically equivalent: quantity ofv photographicdeveloping agent.

2. A process as in claim 1 wherein said photographic developing agent isp-hydroxyphenyl amino acetic acid.

HAROLD OWEN DICKINSON.

REFERENCES CITED The following-references are of record in the file,ofthis patent:

UNJZTEE' STATES PATENTS

1. PROCESS FOR THE PRODUCTION OF COLLOIDAL SILVER WHICH COMPRISESREACTING IN AN AQUEOUS GELATIN SOLUTION SILVER NITRATE IN THE PROPORTIONOF 1/2 VOLUME OF A 1/10 NORMAL SOLUTION WITH 1/2 VOLUME OF A 1/20 MOLARSOLUTION OF A COMPOUND TAKEN FROM THE GROUP CONSISTING OF THIOUREA ANDSELENOUREA AND ADDING THE COLLOIDAL SILVER SALT FORMED WHICH IS TAKENFROM THE GROUP CONSISTING OF COLLOIDAL SILVER SULFIDE AND COLLOIDALSILVER SELENIDE TO AN AQUEOUS GELATIN SOLUTION CONTAINING A QUANTITY OFSILVER NITRATE EQUIVALENT TO 1 TO 4 VOLUMES OF NORMAL SILVER NITRATESOLUTION PER 100 VOLUMES OF GELATIN SOLUTION AND ADMIXING A CHEMICALLYEQUIVALENT QUANTITY OF PHOTOGRAPHIC DEVELOPING AGENT.